The invention relates to a spur gear camshaft drive for an internal combustion engine including a crankshaft timing gear and a camshaft gear and at least one intermediate gear arranged between the timing gear and the camshaft gear, where the gears of the spur gear camshaft drive are each provided with a hub and a gear ring.
In internal combustion engines having a camshaft drive which consists of a plurality of gears between a crankshaft and a camshaft, a variety of influences result in vibration stresses on the camshaft drive. For example, in a diesel internal combustion engine, a relatively strong but short-lived excitation of the crankshaft takes place during an operating stroke in a cylinder, while virtually no torque acts on the crankshaft in periods between operating strokes. This results in relatively irregular, non-uniform movement of the crankshaft with a comparatively jerky vibratory motion. Tolerances in components of the camshaft drive lead to corresponding clearance irregularities between meshing teeth of the gears, which also results in vibrations of the camshaft. In addition, such clearance irregularities are further intensified because steel gears or camshaft drive components made of steel are frequently used, whereas the crank housing is made of aluminum. Because the materials have different temperature expansion coefficients, the distance between centers of respective gears of the camshaft drive during operation of the internal combustion engine is variable, causing additional clearance irregularities.
These clearance irregularities in the camshaft drive have three kinds of undesirable effects. First, synchronization between the crankshaft and camshaft is hampered, since relative motions between camshaft and crankshaft are possible. In internal combustion engines having an integrated pump-nozzle system, in which the camshaft directly determines injection times, the synchronization problems can result in displacements of injection times and, hence, interference with optimal operation of the internal combustion engine. Second, the irregular motion of the crankshaft is transmitted through the camshaft drive into the camshaft as vibration leading to inaccurate injection and valve actuation times. Third, the clearance irregularities lead to increased noise, which may irritate passengers or a vehicle operator in a vehicle compartment.
In addition, an especially great clearance irregularity between a camshaft gear and the last gear of the camshaft drive meshing with the crankshaft timing gear is often present when multiple gears are used between the crankshaft and the camshaft, because tolerances and inaccuracies of distances between centers accumulate correspondingly.
The above-mentioned vibrations are further intensified by forces acting on the camshaft against its direction of rotation, such as during the actuation of a fuel pump of a pump-nozzle element, where a cam of the camshaft has to overcome a great resistance.
Because of all the disadvantages mentioned above, a camshaft drive by means of gears or spur gears is traditionally not used in passenger cars. A camshaft drive by spur gears is used only when there is a small space between crankshaft and camshaft, i.e., in a bottom-lying camshaft. For reducing clearance irregularities, helical gears are used so that operation is as quiet as possible. Despite this, such spur gear or gear camshaft drives have thus far failed to achieve the comfort, quiet operation and accuracy of a camshaft drive with toothed belt wheels or chain wheels with a roller chain.
Self-study program No. 197, xe2x80x9cDer 1,8 1 Dieselmotor im LT ""97,xe2x80x9d published by Volkswagen AG, Wolfsburg, of August 1997, discloses a spur gear camshaft drive wherein, between an intermediate gear of the camshaft and an intermediate gear of the drive for the camshaft, there is provided a spacer fork in such a way that a specified gear tooth clearance is obtained. There, the intermediate gear of the camshaft is arranged on an adjusting-bearing lever which, for assembly, is capable of swiveling about the axis of rotation of the camshaft gear. For assembly, the spacer fork is linked swiveling to the other intermediate gear. After assembly, the spacer fork is tightened, so that the axis of rotation of the intermediate gear of the camshaft is fixed.
The object of the present invention is to provide an improved spur gear camshaft drive wherein the aforementioned disadvantages are overcome so that vibrations in the spur gear camshaft are reduced as rapidly as possible. In particular, feedback of vibrations from the camshaft into the spur gear camshaft drive is largely prevented.
According to one preferred embodiment of the present invention, there is provided a camshaft gear, a crankshaft timing gear, and at least one intermediate gear, where at least one of the aforementioned is configured in such a way that, if its gear ring is turned against its respective hub at a predetermined angle, a readjusting device provided between the hub and the gear ring will exert a readjusting force against said hub and said gear ring, depending on the torsion exerted. This has the advantage that vibrations between the gear ring and the hub are reduced, without affecting synchronization between crankshaft and camshaft of the internal combustion engine. Accordingly, induced torque peaks on the camshaft and on the injection system drive can be reduced.
In a preferred embodiment, cooperating stops limiting torsion are provided on the hub and the gear ring.
In a further preferred embodiment, the readjusting device is designed in such a way that, at a torque of 80 Nm or more, the stops engage one another.
In an especially preferred embodiment, the readjusting device comprises at least one spring, in particular a flat spring, where the springs are designed for example U-shaped and are arranged spaced at equal distances apart, particularly in the peripheral direction, between hub and gear ring. For this purpose, there are provided recesses at equal distances apart in the peripheral direction accommodating the springs in the hub and an annular cover is provided for holding the springs in the recesses and for closing off the recesses in the hub. The cover may be bolted on.
In a further preferred embodiment, for optimal damping of especially high-frequency vibrations, elevations between two springs are provided on an inner periphery of the gear ring, in such a way that the springs exert a readjusting force on the elevations, upon turning of gear ring and hub against one another, as soon as a stop of the hub moves out of a mid-position between two adjacent stops on the gear ring.
In a further especially preferred embodiment, there is arranged between the crankshaft timing gear and the camshaft gear at least one equalizing intermediate gear, whose axis of rotation is freely movable with regard to the crank housing. In this manner, the movable equalizing intermediate gear compensates for varying spacing of axes of rotation between two adjacent gears, by a corresponding relative motion of the axis of the equalizing intermediate gear, so that no clearance irregularity appears even in case of varying spacing of the axes of the adjacent gears. This allows control times to be maintained more accurately by the camshaft and provides quieter operation of the spur gear camshaft drive.
In a preferred embodiment, the axis of rotation of the equalizing intermediate gear is held capable of swiveling by at least two brackets, each of which is supported capable of swiveling at ends facing the axis of the equalizing intermediate gear on fixed axes of gears adjacent to the equalizing intermediate gear. An equalization of clearance directly on the camshaft is obtained, in that the equalizing intermediate gear is arranged between the camshaft gear and a last intermediate gear of the spur gear camshaft drive. Because the entire clearance of the spur gear camshaft drive accumulates at this point, maximum clearance equalization is thereby obtained.
In another preferred embodiment, a transmission ratio of the crankshaft to the camshaft of 2:1 is advantageously obtained by ensuring that the ratio of the number of gear teeth between the equalizing intermediate gear and an adjacent intermediate gear is appropriately selected. Here, the adjacent intermediate gear is, for example, a crankshaft-side intermediate gear.
The invention is described in detail below by reference to the accompanying drawings of systems, which illustrate preferred embodiments of the present invention and serve to explain the principles of the present invention. The drawings are incorporated and constitute part of this disclosure. The scope of the invention is pointed out in the appended claims.